1. Field of the Invention
The invention relates to a method for automatic detection of three-dimensional defects in moving surfaces by means of color vision systems and to an apparatus for performing the method. In particular the invention relates to automatic, real-time inspection of surfaces with respect to defects of different kind and degree, more particular to automatic inspection of moving webs or conveyed piece goods, in which three-dimensional defects, such as indentations, steps, material break-outs, waviness, or dimples, are to be detected, and simultaneously therewith also planar defects defined by color features, texture features and/or geometric features.
2. Description of Prior Art
There is a plurality of vision systems in use for automatic inspection of moving surfaces, e.g. for automatic inspection of paper webs, foils and films, fabrics, steel strips or wood. Most of the systems apply CCD line cameras (gray scale or color) as the sensor for image acquisition, laser scanners being used less frequently. These systems are capturing images, representing e.g. the reflection properties of the surface to be inspected, but containing no direct information on the three-dimensional properties thereof. However, it is generally necessary to detect especially such defects that are characterized by three-dimensional features, such as e.g. indentations, scratches, material break-outs or rough spots. In commonly used systems special techniques for illumination are applied in order to enhance the visibility of three dimensional defects, such that these defects can be detected automatically. However, surfaces in general do not have only three-dimensional defects, but also planar defects, such as stains or discolorations. The images obtained by standard systems do not contain information for distinguishing between planar defects and three-dimensional defects. Therefore in any case a distinction is possible only by means of previous knowledge with respect e.g. to the position or the appearance of different kinds of defects. This procedure fails when the previous knowledge of man cannot be exploited technically or when planar defects may have the same appearance in the image as three-dimensional defects. In such cases, it will be necessary to have information in the image that is in direct relation with the three-dimensional properties of the surface.
For acquisition of three-dimensional information, there is a multiplicity of methods available. These methods can be grouped in two classes: methods giving depth information directly due to their measuring principle (e.g. light section techniques or laser travel-time measurement) and methods exploiting the fact that the reflectance properties of a surface element are dependent upon the orientation and smoothness of the same. The latter methods are referred to as stray light methods. These include a measurement method that is utilized in conjunction with laser scanners. In this method, the surface is scanned with a laser beam, and the reflected light is observed simultaneously from various directions. By comparison of the signals measured in the various channels of observation, it is then possible e.g. to conclude whether the surface element observed is planar or inclined, i.e. whether it is part of a flawless region of a planar, smooth surface or part of the edge of a three-dimensional defect.
Disadvantages of the presently known methods of detecting three-dimensional information in automatic inspection of surfaces are as follows:
Methods for the direct measurement of depth information can be used in surface inspection in exceptional cases only: for most of the applications they are too slow and/or have a too low spatial resolution and/or height resolution. PA1 Laser scanners having a plurality of receiving channels are too expensive for most applications. PA1 For inspection of piece goods, there are modifications of the stray light method in which several images are taken of the test specimen, where the object is being illuminated from different directions for subsequent images.
From the obtained image series it is possible to derive knowledge on the reflectance behavior and the spatial orientation of the surface elements. A prerequisite for employing this method is that the test specimen is at rest. This prerequisite is not fulfilled in applications for automatic inspection of webs, steel or other moving surfaces.